The effects of an ethanol extract of Ferula sinaica roots on the smooth muscles of rabbit and guinea pig were tested in vitro using isolated segments of intestine, trachea and aorta. The extract inhibited the spontaneous movements of rabbit jejunum and guinea pig ileum and acetylcholine-induced contractions. The extract also inhibited the contractions of rabbit tracheal smooth muscle induced by acetylcholine stimulation and the contractions of guinea pig tracheal smooth muscle induced by histamine stimulation. Furthermore, the extract inhibited the contractions of rabbit aorta induced by norepinephrine stimulation. These inhibitions were concentration-dependent and reversible.